Merge pull request #578 from maxim-belkin/fix-404

{% comment %}

This is adapted from: https://ricostacruz.com/til/relative-paths-in-jekyll

{%- comment -%}

When the website is built by GitHub Pages,

'site.url' is set to 'https://username.github.io'

'site.baseurl' is set to '/lesson-name'

`page.url` gives the URL of the current page with a leading /:

When we start a local server using `jekyll serve`,

'site.url' is set to 'http://localhost:4000' and

'site.baseurl' is empty.

- when the URL ends with the extension (e.g., /foo/bar.html) then we can get

  the depth by counting the number of / and remove - 1

- when the URL ends with a / (e.g. /foo/bar/) then the number / gives the depth

  directly

{% endcomment %}

In both of the above cases we set 'relative_root_path' to 'site.url + site.baseurl'.

{% assign relative_root_path = '' %}

When we build a website locally with `jekyll build`,

both 'site.url' and 'site.baseurl' are empty.

This case is handled by the last 'else' in the code below.

The logic there follows the (adapted) instructions found at:

    https://ricostacruz.com/til/relative-paths-in-jekyll

{% assign last_char = page.url | slice: -1 %}

    `page.url` gives the URL of the current page with a leading /:

    - when the URL ends with an extension (e.g., /foo/bar.html),

      we can get the 'depth' of the page by counting the number of

      forward slashes ('/') and subtracting 1

    - when the URL ends with a forward slash (e.g. /foo/bar/),

      we can get the depth of the page by counting the number of /

{%- endcomment -%}

{% if last_char == "/"}

{% if site.url %}

  {% assign relative_root_path = site.url | append: site.baseurl %}

{% else %}

  {% assign last_char = page.url | slice: -1 %}

  {% if last_char == "/" %}

    {% assign offset = 0 %}

  {% else %}

    {% assign offset = 1 %}

  {% endif %}

  {% assign depth = page.url | split: '/' | size | minus: offset %}

  {% if    depth <= 1 %}{% assign relative_root_path = '.' %}

  {% elsif depth == 2 %}{% assign relative_root_path = '..' %}

{% elsif depth == 3 %}{% assign relative_root_path = '../..' %}

{% elsif depth == 4 %}{% assign relative_root_path = '../../..' %}

  {% else %}{% capture relative_root_path %}..{% for i in (3..depth) %}/..{% endfor %}{% endcapture %}

  {% endif %}

{% endif %}